But not all regex flavors support negative lookbehind. And those that do typically have severe restrictions on the lookbehind, eg, it must be a simple fixed-length expression. To avoid incompatibility, we can restate our solution using negative lookahead:

(?!fox\b)\b\w+

You can test this regex in the cool online JavaScript Regex evaluator. Unfortunately, JavaScript doesn’t support negative lookbehind, so if you want to test that one, I recommend RegexBuddy. It’s not free, but it’s the best regex tool out there by far– and it keeps getting better with every incremental release.